1. Field of the Invention
The present invention relates to a control device and a control method for an automatic transmission, and in particular relates to a control device and a control method for an automatic transmission, which are effective with regard to failure countermeasures during garage shifting.
2. Description of the Related Art
Among types of automatic transmission mounted to a vehicle, there is a stepped type of automatic transmission which comprises a fluid coupling such as a torque converter or the like and a planetary gear type speed reduction device; and also there is a stepless type of automatic transmission which comprises a metallic belt wound between two pulleys whose effective diameters can be changed by oil pressure.
A stepped type automatic transmission is connected to the engine via a fluid coupling such as a torque converter or the like. Such a stepped type of automatic transmission comprises a transmission construction (a planetary gear type speed reduction device construction) which provides a plurality of power transmission paths, and, for example, changes over between these power transmission paths automatically based upon the accelerator opening amount and the vehicle speed; in other words, it is built so as to perform changeover between its various speed change ratios (running speed ranges) automatically. With this stepped type of automatic transmission, the engaged gear stage is determined by engaging and releasing predetermined combinations of clutch elements and brake elements (which are friction elements) and one way clutch elements.
On the other hand, a stepless type automatic transmission is also connected to the engine via a fluid coupling such as a torque converter or the like. For example, a belt type stepless transmission employs a metallic belt and a pair of pulleys, and implements continuous stepless speed changing by varying the effective diameters of these pulleys by oil pressure actuation. In more detail, this endless metallic belt is passed around an input side pulley, which is fixed to an input shaft, and an output side pulley, which is fixed to an output shaft. This input side pulley and output side pulley each comprises a pair of sheaves the width of the groove between with can be changed in a stepless manner, and, by thus changing the groove widths of the pulleys, the radiuses of the portions of the endless metallic belt which pass around the input side pulley and the output side pulley change, so that the rotational speed ratio between the input side pulley and the output side pulley, in other words the speed change ratio, can be changed continuously and steplessly.
With either of these types of automatic transmission, generally, a shift lever is provided to the vehicle in which the automatic transmission is fitted, this shift lever being actuated by the driver of the vehicle; and speed range positions (for example a reverse running position, a neutral position, and one or more forward running positions) are set based upon actuation of this shift lever.
In order to take a vehicle to which an automatic transmission having this type of structure is mounted out of its garage before driving the vehicle upon the road, or in order to put the vehicle back into the garage after driving the vehicle upon the road, or the like, so called garage shifting is performed, in which the shift lever is shifted from the parking (P) position to the reverse running (R) position in order to drive the vehicle backwards, or from the neutral (N) position to the forward running (D) position or to the reverse running (R) position, in order to move the vehicle forwards or backwards respectively.
In consideration of the possibility that the shift position for shift actuation, including such garage shift actuation, may not be accurately detected, for example, in Japanese Patent Application Publication No. JP-5-306763, there is disclosed a shift position decision control device which, when the N position and the L position have been detected at the same time, gives priority to the detection of the L position.
This shift position decision control device is a shift position decision control device of an automatic transmission for a vehicle which includes a shift position detection means which detects the shift position set by a shift actuation mechanism electrically, and a means which operates a friction engaging element for engine braking by changing over an oil conduit within an oil pressure circuit, based upon an electrical detection signal for the shift position; and it includes: a decision means which decides whether or not a non-running position is being detected by the shift position detection means; a decision means which decides whether or not a shift position in which engine braking must be put into effect is detected by the shift position detection means; and an operating means which, when even in the state in which a non-running position is detected, also at the same time a shift position in which engine braking must be put into effect is detected, operates the friction engaging element for engine braking, by giving priority to the detection of the shift position in which engine braking must be put into effect.
With this shift position decision control device for an automatic transmission for a vehicle, even in the state in which a non-running position (the N position) is detected, when at the same time a shift position in which engine braking must be put into effect (the L position) is detected, it is arranged to give priority to the detection of this shift position in which engine braking must be put into effect, and, based upon this detection, the friction engaging element for engine braking is operated. As a result, the implementation of engine braking is not deteriorated.
It is included in the disclosure of the above document that shift operation is detected as a shift position by an electrical signal, and, based thereupon, the gear speed change of the automatic transmission (if it is of a stepped type), or the speed change gear ratio of the automatic transmission (if it is of a stepless type or of a stepless type which incorporates a sequential shift mechanism), is changed.
Now, when a fault occurs in the sensor (the switch) which detects this shift position, the driver is not able to engage the shift range to which he has shifted the shift lever. In particular, there is the possibility that it is not possible to start the vehicle off from rest, if the shift operation from the N position to the D position (garage shifting) cannot be detected.
However, in the above described Japanese Patent Application Publication No. JP-5-306763, no mention is made of this type of problem. Furthermore when, with the shift lever being in its D position, it is detected (erroneously) that the shift lever is still in its N position, when the driver depresses the accelerator pedal and demands that the vehicle should start off from rest, a clutch (for example a clutch C1 for starting off from rest) which is a friction engaging element is abruptly engaged, so that, from the state in which the N position is engaged, abruptly the first speed range is engaged. At this time, since there is an inevitable response delay in a linear solenoid valve, in particular at low temperatures, accordingly there is a possibility that the actual oil pressure may exceed the commanded oil pressure, in which case there is a possibility that the control may become unstable.